Direct peer-to-peer device-to-device (D2D) communication can be exploited in cellular networks to improve overall network capacity as well as mitigate coverage holes for user equipment (UEs) that do not have network coverage.
D2D communication may involve bidirectional communication, where both devices receive and transmit in the same or different resources. D2D communication may also involve unidirectional communication, in which one of the devices transmits signals and the other device receives the signals. There may also exist a point-to-multipoint (e.g., multicast, broadcast, etc.) scenario in which a plurality of devices receive signals from the same transmitting device. The point-to-multipoint scenario is particularly useful for emergency services or public safety operation to spread vital information to several devices in an affected area. The term D2D communication and D2D operation are interchangeably used.
Typically, devices operate under the supervision of a radio access network with radio access nodes (e.g., a base station). In some scenarios, the devices themselves establish direct communication without the intervention of the network infrastructure.
In cellular network assisted D2D communications (or simply, network assisted D2D communications), UEs in the vicinity of each other can establish a direct radio link (D2D bearer). While UEs communicate over the D2D “direct” bearer, they also maintain a cellular connection with their respective serving base station (eNB). This direct link is interchangeably called a network (NW) link, D2D-NW link, or by other names equally descriptive. The NW link is used, for example, for resource assignment for D2D communication, maintenance of radio link quality of D2D communication link, or any other suitable parameter.
There are a variety of potential coverage scenarios for D2D communication. Examples of the various coverage scenarios are described in more detail below with respect to FIGS. 1A-C.
FIG. 1A is a schematic diagram of a partial-coverage scenario for D2D communication. More particularly, FIG. 1 A illustrates UEs 110A and 110B and network node 115. In the partial-coverage scenario, at least one D2D UE communicating is under the network coverage, and at least one UE communicating is not under the network coverage. For example, in the scenario illustrated in FIG. 1A, UE 110B is under network coverage (i.e., within the coverage area of network node 115), and UE 110A is not under the network coverage. As described above, the D2D UE 110A not receiving network coverage can be due to lack of a network node in its vicinity, due to insufficient resources in any of the network nodes in its vicinity, or for other reasons. The partial-coverage scenario is also interchangeably called partial-network (PN) coverage.
FIG. 1B is a schematic diagram of the in-coverage scenario for D2D communication. More particularly, FIG. 1B illustrates UEs 110A and 110B and network node 115. In the in-coverage scenario, all D2D UEs communicating are under the network coverage. For example, in the scenario illustrated in FIG. 1B, both UE 110A and UE 110B are under network coverage (i.e., within the coverage area of network node 115). The D2D UEs 110A and 110B can receive signals from and/or transmit signals to at least one network node 115. In this case, the D2D UEs 110A and 110B can maintain a communication link with the network. The network in turn can ensure that the D2D communication does not cause unnecessary interference. The in-coverage scenario is also interchangeably called in-network (IN) coverage.
FIG. 1C is a schematic diagram of an out-of-coverage scenario for D2D communication. More particularly, FIG. 1C illustrates UEs 110A and 110B. In the out-of-coverage scenario, D2D UEs 110A and 110B communicating with each other are not under network node coverage. D2D UEs 110A and 110B cannot receive signals from and/or transmit signals to any of the network nodes. Typically, the lack of coverage is due to complete absence of network coverage in the vicinity of D2D UEs 110A and 110B. The lack of coverage, however, may also be due to insufficient resources in the network nodes to serve or manage D2D UEs 110A and 110B. Therefore, in this scenario the network cannot provide any assistance to the devices. The out-of-coverage scenario is also interchangeably called out-of-network (OON) coverage.
The emissions outside the bandwidth or frequency band of a UE are often termed as out-of-band (OOB) emissions or unwanted emissions. The major OOB and spurious emission requirements are typically specified by the standard bodies, and eventually enforced by the regulators in different countries and regions for both UEs and base stations. Examples of the OOB emissions include Adjacent Channel Leakage Ratio (ACLR) and Spectrum Emission Mask (SEM). Typically, the OOB emission requirements ensure that the emission levels outside the transmitter channel bandwidth or operating band remain several tens of dB below the transmitted signal.
Conservation of UE battery power can be facilitated when the UE has an efficient power amplifier (PA). The PA can be designed for certain operating points or configurations or set of parameter settings, such as, for example, modulation type, number of active physical channels (e.g., resource blocks in E-UTRA or number of CDMA channelization codes code and/or spreading factor in UTRA). To ensure that a UE fulfills OOB/spurious requirements for all allowed uplink (UL) transmission configurations, the UE is allowed to reduce its maximum UL transmission power in some scenarios. This is called maximum power reduction (MPR) or UE power back-off in some literature. For instance, a UE with maximum transmit power of 24 dBm power class may reduce its maximum power from 24 dBm to 23 or 22 dBm, depending upon the configuration.
In E-UTRA, an additional MPR (A-MPR) for the UE transmitter has also been specified in addition to the normal MPR. The A-MPR can vary between different cells, operating frequency bands and more specifically between cells deployed in different location areas or regions. In particular, the A-MPR may be applied by the UE in order to meet the additional emission requirements imposed by the regional regulatory organization. A-MPR is an optional feature, that is used by the network when needed depending upon the co-existence scenario. The A-MPR defines the UE maximum output power reduction (on top of the normal MPR) needed to fulfill certain emission requirements by accounting for factors such as: bandwidth, frequency band or resource block allocation. The A-MPR is therefore controlled by the network node by signaling to the UE a parameter called the network signaling (NS) parameter. For example, NS_01 and NS_02 correspond to different levels of pre-defined A-MPRs.
Even in the case of network-assisted D2D communication, the network may not fully manage the interference. Therefore there exists the potential for D2D communications to cause interference to both serving cellular networks as well as legacy co-located networks or co-existing networks in the same geographical region.
In LTE, potential D2D interference can be intra-frequency co-channel interference (i.e., collisions between transmitted resource blocks (RBs) within the system bandwidth), and/or interference from in-band emissions from the transmitting RBs within the system bandwidth into adjacent RBs to those RBs being employed for the desired transmission. Additionally, D2D communications can result in inter-device and intra-device interference across a number of channels in LTE including, for example, the Physical Uplink Control Channel (PUCCH) and Physical Uplink Shared Channel (PUSCH). The D2D communication typically takes place over LTE uplink channels, such as PUCCH/PUSCH or similar channels.
There also exists the potential for D2D communications to cause interference to both serving cellular networks as well as legacy networks, especially legacy networks that are co-located with the serving cellular networks. The interference may also be caused to the networks that co-exist in the same geographical areas where D2D UEs operate.
FIG. 2 is a schematic diagram of D2D transmission interference. More particularly, FIG. 2 illustrates UEs 110A-C and network nodes 115A and 115B. One or more of the UEs may be D2D capable. For example, UEs 110B and 110C may be D2D UEs. Transmission 205 from D2D UE 110B to D2D UE 110C may be a desired D2D transmission.
In FIG. 2, the D2D transmission 205 acts as an aggressor or interferer 215 to desired LTE transmissions on the UL for the D2D UE being out-of-network coverage and in-network or partial coverage. For example, transmission 210 from UE 110A to network node 115B may be interfered with by D2D communication 205. Note that these interference scenarios can only occur when the LTE network is operating in TDD duplex mode and the D2D transmission is not synchronized to the LTE network. For an FDD LTE network, since the D2D transmissions are on the UL, no co-channel interference will occur on the FDD DL channel. Interference to co-located co-existing networks, however, can occur.
The interfering situation becomes worse when D2D UEs are in partial-network coverage or even worse when they are completely out of network coverage. The following problems may occur: performance may be severely degraded; the D2D communication may not be sustained; and/or regulatory requirements on radio emissions may not be met by the D2D UEs.